It will be convenient to describe the preferred embodiments of the invention using terminology consistent with that used by the 3rd Generation Partnership Project (3GPP). However, the present invention should not be considered as being limited to use in networks complying with 3GPP Standards.
The applicant does not concede that the prior art discussed herein forms part of the common general knowledge in the art at the priority date of the present application.
Throughout the present specification the following abbreviations and acronyms will be used.
RSReference SignalCEChannel EstimationREResource ElementCIRChannel Impulse ResponseVCIRVirtual CIRCFRChannel Frequency ResponseVCFRVirtual CFRTFTime FrequencyCPCyclic PrefixABSAbsolute ValueFHFrequency Hopping of RSUEUser EquipmentLTELong Term Evolution of 3GPP networks
In OFDM system, pilot symbol or RS are inserted into the time frequency (TF) grid in a transmitted signal so that UE can estimate the channel for all RE carrying data. To keep the overhead relating to RS small, RS RE are spaced in time and frequency as shown in FIG. 1 below.
In FIG. 1:
There are 7 OFDM symbols in one slot in time direction
There are 17 sub-carriers in frequency direction
1st and 5th OFDM symbols in a slot carry RS
Diagonally hatched and cross hatched RE contains RS
RS spacing in the frequency domain is 6.
There exist various way of completing the CE for the rest of the RE. In a typical method, the following steps can be followed:
UE first obtains CE for every sub-carrier in the OFDM symbol where RS is transmitted, e.g. the 1st and 5th OFMD symbol in a slot in this example; (see block M1 to M6 below).
Then UE uses time interpolation to obtain CE for every OFDM symbol in a slot where RS is not transmitted, e.g. 2nd, 3rd, 4th, 6th, 7th OFDM symbol in a slot in this example (see block M7 below).
The basic system for performing CE in a UE is illustrated in FIG. 2. Its operation will now be briefly described:
In this discussion let Nrs denote the number of RS RE in one OFDM symbol
Block M1—uses Nrs RS REs to generate a vector of tentative estimates, the Length of the vector Nls=Nrs.
Block M2—generates vector of VCFR of size N which is the smallest power of 2 that is larger Nls, using the vector from block M1 as an input. The simplest way is to add (N-Nls) zeros to the end of the input vector.
Block M3—performs an N point IFFT
Block M4—performs de-noising of the time domain signal i.e. zeroing all samples which have magnitude smaller than a preset de-noise threshold.
Block M5—performs an N points FFT
Block M6—performs frequency interpolation between RS RE to obtain CE for all sub-carriers (6Nrs) in the OFDM symbol.
Block M7—performs time interpolation to obtain CE for all OFDM symbol in a slot not carrying RS
However using such a system it may not be possible to obtain accurate CE in different channel conditions, mainly defined by Doppler, Delay spread, and Noise level.